linux-kernel - Re: [PATCH] arm64: Add the arm64.nolse

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Message-ID: <ZLBLwG2LJ4gZLfbh@FVFF77S0Q05N.cambridge.arm.com>
Date:   Thu, 13 Jul 2023 20:08:48 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     "Aiqun(Maria) Yu" <quic_aiquny@...cinc.com>
Cc:     Will Deacon <will@...nel.org>, corbet@....net,
        catalin.marinas@....com, maz@...nel.org, quic_pkondeti@...cinc.com,
        quic_kaushalk@...cinc.com, quic_satyap@...cinc.com,
        quic_shashim@...cinc.com, quic_songxue@...cinc.com,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm64: Add the arm64.nolse_atomics command line option

On Thu, Jul 13, 2023 at 10:08:34PM +0800, Aiqun(Maria) Yu wrote:
> On 7/13/2023 7:20 PM, Mark Rutland wrote:
> > Are you saying that LSE atomics to *cacheable* mappings do not work on your
> > system?
> > 
> > Specifically, when using a Normal Inner-Shareable Inner-Writeback
> > Outer-Writeback mapping, do the LSE atomics work or not work?
> *cacheable* mapping have the LSE atomic is not working if far atomic is
> performed.

Thanks for confirming; the fact that this doesn't work on *cacheable* memory is
definitely a major issue. I think everyone is confused here because of the
earlier mention of non-cachable accesses (which don't matter).

I know that some CPU implementations have EL3 control bits to force LSE atomics
to be performed near (e.g. in Cortex-A55, the CPUECTLR.ATOM control bits),
which would avoid the issue while still allowing the LSE atomics to be used.

If those can be configured in EL3 firmware, that would be a preferable
workaround.

Can you say which CPUs are integrated in this system? and/or can you check if
such control bits exist?

Thanks,
Mark.